They are referring to the younger players on defense. Supposedly early in the year they just believed they would over come whatever happened. Then I think after the Jets destruction they lost some faith.
An excerpt:
The level of adversity might be new for a lot of the young players Buffalo’s defense depends on. For young players who were accustomed to doing a lot of winning in college, a three-game losing streak might be unfamiliar territory for them.
That’s why the vets are working hard to re-establish that mentally tough mindset with their younger teammates, especially in instances where there might be a big play made against them